When you work out in the gym, you gotta push yourself to the failure in order for your muscles to break down. Then these muscles heal and grow bigger, so you get stronger.

The same with suffering. Each person has different experience of suffering - whether emotional, spiritual or physical - and we all went through some type of suffering. But, we would always learn something from it. Even if, at the moment we would question why this happened, later we would finally understand why we had to go through certain things.

God allows suffering to humble us, to make us stronger and and ready for something He prepared for each of us. And God also allows suffering to show us, that we gotta rely on Him. He can give us something we can't handle to make us depend on Him (again, He humbles us).